1. Field of the Invention
This invention pertains to radio frequency (RF) antennae, and in particular to RF antennae adapted for short pulse signal transmission, where a first ground plane monopole antenna generates a short period pulse with minimal residual signal after application of transmission energy to the antenna has ceased, and where a second ground plane monopole antenna receives backscatter containing more useful information and less noise than penetrating radar systems in the prior art.
2. Prior Art
Since the discovery of radio frequency transmission, antenna design has been an integral part of many telemetry applications. Antenna applications became more diverse as the potential range of usable transmission frequencies increased, and antenna designs became more exotic. Of particular relevance to the present invention are the antennas operating in the microwave range of frequencies.
Antennas capable of transmitting microwaves have come in many shapes and designs as illustrated by U.S. Pat. Nos. 4,649,396, 4,903,033 and 5,068,671. Not only has each has been designed to operate in the microwave range, they were designed to overcome specific problems, such as transmitting in specific environments such as high winds, transmitting specific types of polarized signals, and transmitting broad-band signals having desirable phase and polarization characteristics respectively.
The variety of shapes and configurations of the antenna elements demonstrates that solutions to specific problems often require specific antenna geometries. For example, U.S. Pat. No. 4,649,396 discloses a monopole antenna mounted perpendicular to a ground plane. That particular antenna is known to those skilled in the art as a ground plane antenna. A ground plane antenna is a capacitive structure, unlike other antenna elements that characteristically have current flow. The specific structure can vary greatly, but must have two distinguishing elements. First, it must have a ground plane which is any surface or plane creating configuration that assists in establishing the radiation pattern of the antenna element, and second, it must have a radiating element that is typically a fraction of the wavelength to be transmitted. Geometries for a ground plane antenna are shaped to provide specific transmitting and receiving characteristics, limited only by the creativity of the designer.
When designing an antenna, a fundamental consideration is how that shape will vibrate. An antenna functions because of the principle of resonance. An antenna element resonates at the frequency applied by a transmission source connected to the element, or at the frequency of a received signal. While resonance of the antenna element is desired, uncontrolled resonance only serves to complicate certain applications of radio frequency technology, such as radar. The uncontrolled resonance being referred to is any resonance of the antenna element that occurs after a transmission signal is applied and subsequently terminated.
As might be expected, all the antennas cited in the U.S. patents above resonate for a relatively short period of time after transmission of an applied signal has stopped. It is important to note that the time frame being discussed is only a matter of nanoseconds, and is therefore usually inconsequential for most applications. However, when working with radar signals in the range of microwaves that must penetrate a mass of differing dielectric materials, nanoseconds are critical. The end result of using the antennas disclosed in the prior art is that their effective application for radar in a cluttered dielectric environment is considerably reduced.
The reduction in effectiveness of antenna elements because of resonance would seem to be inconsistent with an antennas' principle of operation. However, it is only because the frequency of operation of the present invention is in the range of microwaves that the dichotomy becomes apparent. The prior art antennas tested for use in the present invention characteristically produced a trailing resonance signal of mere nanoseconds of duration after the transmission source was eliminated. The problem arises because the duration of the signal transmitted is also in the range of nanoseconds. Thus, noise generated by the antenna itself is similar to the transmission signal being generated, making interpretation of backscatter more difficult and interfering with operation of the antenna as a penetrating radar.
A radar attempts to gather information from backscatter. Backscatter is the reflected signal bounced off objects of interest. When the signal transmitted is of a known amplitude, frequency and duration, it is easier to learn from backscatter, and to determine characteristics about the object reflecting the signal. In effect, it becomes easier to separate useful information from the noise. However, when the signal transmitted is followed by a trailing resonance signal as is typical in the prior art, backscatter might be a signal reflected from a waveform of unknown amplitude, frequency, or duration. Determining which backscatter signals contain useful information becomes complicated, and often sophisticated and costly equipment is required to analyze all of the backscatter to find the desired information.
Accordingly, the challenge in designing a microwave radar antenna is in overcoming trailing transmission resonance, producing a single cycle uniform output of known amplitude, frequency and duration, and receiving useful backscatter that is not affected by transmission resonance of the transmitted pulse.